This invention relates to switching frequency ripples in coils, and, more particularly, to methods and apparatus, which eliminate and/or reduce a switching frequency ripple in the coil voltage and current.
Magnetic resonance imaging systems require very powerful amplifiers, typically several hundred amperes and more than 1500 volts, to drive gradient coils that provide gradient fields for the slice, frequency and phase encoding used in the imaging process. The large currents and speed desired in newer systems made conventional linear amplifiers impractical, requiring the use of switching amplifiers that have significantly reduced losses. One such amplifier is described in U.S. Pat. No. 6,031,746. One drawback of a switched amplifier is the output noise because the output is a pulsating voltage waveform pulse width modulated (PWM). The PWM output voltage results in current ripple in the gradient coils which can affect imaging quality and resolution, and large voltage variation (dv/dt) across the gradient coils can result in coupled noise to other parts of the system, which also has detrimental effects on the image quality. It is therefore desirable to reduce or eliminate such switching frequency ripples.
One difficulty to obtain a low ripple with conventional filtering is that the filter elements required would be very large and with considerable losses due to the large currents and the damping requirements. For example, if a single stage LC filter is used to reduce ripple, the attenuation desired might result in a filter bandwidth requirement that would not permit the amplifier to operate with the desired output bandwidth of several kilohertz. In addition to that, the series inductor of the LC filter should be capable of carrying several hundred amps and the damping resistor used for the capacitor will dissipate several hundred watts.
Earlier amplifiers used to drive gradient coils were linear amplifiers (Class A amplifiers) that provided a desired output current and voltage with transistors operating in their active region, thus with relatively large voltage drop. The large voltage drop together with the large output currents, several hundred amps, result in such high losses that make this approach impractical for high performance systems. Initial improvements to those linear amplifiers were to supply them with a voltage regulated according to the output voltage requirements, which attempted to reduce the voltage drop in the output stage transistors operating in the active mode (as described in U.S. Pat. Nos. 5,663,647 and 5,105,153).
The output stage losses are still quite high, and a bus regulator that is usually a high-efficiency switching regulator has problems similar to the ones for the switching amplifier. The bus regulator has to be able to provide a desired bandwidth and a very low output ripple to avoid this ripple from showing up in the output waveform. This limits the application of those approaches to reduce the ripple only when there are no large current variations, like on the top of trapezoidal current waveforms. Another approach has been to use a resonant stage before the linear output amplifier to boost the supply voltage to the amplifier only when large current transitions were required at the output (as described in U.S. Pat. Nos. 5,245,287, 5,298,863, and 5,617,030). These solutions were especially applicable when desired output waveforms have trapezoidal shape. One drawback in these approaches is that the output stage still has to correct the shape of the output waveform during the transition, because usually a sinusoidal transition is not desired, resulting in additional losses, and more importantly, they are not practical for some imaging waveforms like spirals or sinusoids that require a continuously changing voltage, because in this last case, the single resonant transition would not be usable.
One common method used for switching amplifiers is to use multiple stages of filtering to achieve the desired attenuation. This approach has two main disadvantages. First, the multiple stages introduce a large phase shift from filter input to output that makes it difficult to achieve stable tight output control with the desired high bandwidth. If multiple stage filtering is used, it is difficult to obtain a reasonable power converter bandwidth while maintaining stability due to the multiple stages of phase shifting introduced by the filters. Second, to avoid voltage ringing across capacitors in each stage the filter is damped with resistors that reduce the filter effectiveness and that are quite large because the losses reach several hundred watts in most cases.